Precision liquid metering and dispensing apparatus in the field utilize various devices to meter liquids precisely in a dispensing process. Mechanisms used to meter liquids in microliter or milliliter quantities include time per pressure liquid displacement systems, positive liquid displacement pumps, peristaltic tubing pumps, and others. The positive displacement pump is known to be the most accurate and robust device for use in small volume dispensing applications.
There are various positive displacement devices which can be used to move and deposit liquids in a dispensing process. They include diaphragm pumps, piston pumps, gear pumps, syringe pumps and various other mechanisms. The ceramic rotary reciprocating piston pump is the mechanism which is the basis of this invention.
A ceramic rotary reciprocating piston pump offers several advantages over other positive displacement liquid dispensers. One of the advantages results from using the rotary motion of the piston with an integral valving feature to perform the valve function. This method of valving a pump is advantageous in that it eliminates peripheral valving mechanisms that can slow the cycling time of the pump or otherwise be a detriment to the pump's performance. Additionally, the use of a thermally and mechanically stable ceramic material in the construction of the piston and cylinder permits an extremely close running fit to be created thus eliminating the need for secondary seals. The resulting system offers excellent repeatability and long-term reliability as a result of its simplicity of design and limited use of moving parts.
Traditional displacement adjustment of a ceramic rotary reciprocating piston pump utilizes an angularly offset drive. This method allows the magnitude of the piston stroke to be changed by adjusting the relative angular relationship of the piston to the driving motor and its output spindle. U.S. Pat. No. 3,168,872 to H. E. Pinkerton issued Feb. 9, 1965 and entitled "POSITIVE DISPLACEMENT PISTON PUMP" is exemplary of a rotary reciprocating piston pump utilizing an angularly offset drive. The pump of this patent employs a ducted piston which reciprocates and rotates synchronously in a bi-ported cylinder. The piston duct is arranged to connect the ports alternately with the pumping chamber. One port communicates with the pumping chamber on the down stroke of the piston, while the other port is arranged to be exposed to the chamber on the piston upstroke. A piston-cylinder assembly is coupled to the output of a drive motor through an interposed collar or yoke. The piston includes at its outer end a laterally projecting arm having a ball bearing which is adapted to ride in a socket in the collar to thereby provide a universal joint between these parts. A cylinder conveniently receives the piston and is mounted on a bracket rotatable about a vertical axis. The cylinder is provided with at least one pair of ports both of which communicate with the cylinder pumping chamber. When the axis of the collar and that of the piston and cylinder are substantially coaxial, the piston does not reciprocate in the cylinder during the rotation of the collar. As such, no pumping action occurs. When the cylinder is angled about its pivot, the piston will reciprocate at an amount proportional to the angular displacement. The direction of rotation, that is either clockwise or counterclockwise determines the direction of fluid feed. The magnitude of the angular displacement of the piston and cylinder determines the amplitude of piston stroke and consequently flow rate. In a variation, the yoke rather than the cylinder is pivotal. In the past, the adjustment of the angular relationship of the piston to the driving motor and output spindle, collar or yoke is accomplished with a threaded mechanism such as a micrometer. It should be noted that in priming and purging of air from a liquid metering apparatus of this type maximizing the piston's stroke is advantageous. In addition, a long piston stroke provides increased liquid turbulence within the pumping chamber, a proven benefit for clean in place systems. In order to achieve a long piston stroke, the angular relationship of the piston to the drive spindle must be increased to its maximum limit. After successfully priming or cleaning the pumping apparatus, a time consuming adjustment and calibration procedure is required to restore the pump's output to a desired volumetric displacement.
Traditional rotary reciprocating pump designs accelerate the liquids they are displacing in a manner fixed by the mechanical relationship of the pump to the drive motor and spindle. The displacement of liquid by the piston is a cosine function with the velocity of the liquid at the beginning and end of the intake and discharge strokes being zero. As a result of this velocity profile, the pumping apparatus is unable to eject small volumes of liquid from the dispensing tip.
It is therefore the primary object of the present invention to provide a rotary reciprocating pumping apparatus with a positive two position adjustment feature which will allow the piston's stroke to be preferably automatically increased to a maximum and repeatably, preferably automatically returned to a second, calibrated dispensing position, thereby eliminating time consuming adjustments required with traditional rotary reciprocating pump designs.
An additional object of the invention is to provide such rotary reciprocating pumping apparatus with an adjustable liquid displacement velocity profile to achieve an increase in the velocity of the liquid at the end of the pump's discharge cycle to enable ejection of small amounts of liquid from a dispensing tip, thereby eliminating the inaccurate and time consuming operation of "touching off" a small volume drop of liquid characterizing known pumping systems.
It is a further object of the invention to provide a rotary reciprocating pump having enhanced fluid performance with increased ease of use during pump priming and cleaning.
Other objects and advantages will become apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention .